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 uClibc and Glibc are not the same -- there are a number of differences which

may or may not cause you problems.  This document attempts to list these

differences and, when completed, will contain a full list of all relevant

differences.

1) uClibc is smaller than glibc.  We attempt to maintain a glibc compatible

interface, allowing applications that compile with glibc to easily compile with

uClibc.  However, we do not include _everything_ that glibc includes, and

therefore some applications may not compile.  If this happens to you, please

report the failure to the uclibc mailing list, with detailed error messages.

2) uClibc is much more configurable then glibc.  This means that a developer

may have compiled uClibc in such a way that significant amounts of

functionality have been omitted.

3) uClibc does not even attempt to ensure binary compatibility across releases.

When a new version of uClibc is released, you may or may not need to recompile

all your binaries.

4) malloc(0) in glibc returns a valid pointer to something(!?!?) while in

uClibc calling malloc(0) returns a NULL.  The behavior of malloc(0) is listed

as implementation-defined by SuSv3, so both libraries are equally correct.

This difference also applies to realloc(NULL, 0).  I personally feel glibc's

behavior is not particularly safe.  To enable glibc behavior, one has to

explicitly enable the MALLOC_GLIBC_COMPAT option.

4.1) glibc's malloc() implementation has behavior that is tunable via the

MALLOC_CHECK_ environment variable.  This is primarily used to provide extra

malloc debugging features.  These extended malloc debugging features are not

available within uClibc.  There are many good malloc debugging libraries

available for Linux (dmalloc, electric fence, valgrind, etc) that work much

better than the glibc extended malloc debugging.  So our omitting this

functionality from uClibc is not a great loss.

5) uClibc does not provide a database library (libdb).

6) uClibc does not support NSS (/lib/libnss_*), which allows glibc to easily

support various methods of authentication and DNS resolution.  uClibc only

supports flat password files and shadow password files for storing

authentication information.  If you need something more complex than this,

you can compile and install pam.

7) uClibc's libresolv is only a stub.  Some, but not all of the functionality

provided by glibc's libresolv is provided internal to uClibc.  Other functions

are not at all implemented.

8) libnsl provides support for Network Information Service (NIS) which was

originally called "Yellow Pages" or "YP", which is an extension of RPC invented

by Sun to share Unix password files over the network.  I personally think NIS

is an evil abomination and should be avoided.  These days, using ldap is much

more effective mechanism for doing the same thing.  uClibc provides a stub

libnsl, but has no actual support for Network Information Service (NIS).

We therefore, also do not provide any of the headers files provided by glibc

under /usr/include/rpcsvc.   I am open to implementing ldap based password

authentication, but I do not personally intend to implement it (since I have no

use for it).

9) uClibc's locale support is not 100% complete yet.  We are working on it.

10) uClibc's math library only supports long double as inlines, and even

then the long double support is quite limited.

11) uClibc's libcrypt does not support the reentrant crypt_r, setkey_r and

encrypt_r, since these are not required by SuSv3.

12) uClibc directly uses the kernel types to define most opaque data types.

13) uClibc directly uses the linux kernel's arch specific 'stuct stat'.

******************************  Manuel's Notes  ******************************

Some general comments...

The intended target for all my uClibc code is ANSI/ISO C99 and SUSv3

compliance.  While some glibc extensions are present, many will eventually

be configurable.  Also, even when present, the glibc-like extensions may

differ slightly or be more restrictive than the native glibc counterparts.

They are primarily meant to be porting _aides_ and not necessarily

drop-in replacements.

Now for some details...

time functions

--------------

1) Leap seconds are not supported.

2) /etc/timezone and the whole zoneinfo directory tree are not supported.

   To set the timezone, set the TZ environment variable as specified in

   http://www.opengroup.org/onlinepubs/007904975/basedefs/xbd_chap08.html

   or you may also create an /etc/TZ file of a single line, ending with a

   newline, containing the TZ setting.  For example

   echo CST6CDT > /etc/TZ

3) Currently, locale specific eras and alternate digits are not supported.

   They are on my TODO list.

wide char support

-----------------

1) The only multibyte encoding currently supported is UTF-8.  The various

   ISO-8859-* encodings are (optionally) supported.  The internal

   representation of wchar's is assumed to be 31 bit unicode values in

   native endian representation.  Also, the underlying char encoding is

   assumed to match ASCII in the range 0-0x7f.

2) In the next iteration of locale support, I plan to add support for

   (at least some) other multibyte encodings.

locale support

--------------

1) The target for support is SUSv3 locale functionality.  While nl_langinfo

   has been extended, similar to glibc, it only returns values for related

   locale entries.

2) Currently, all SUSv3 libc locale functionality should be implemented

   except for wcsftime and collating item support in regex.

stdio

-----

1) Conversion of large magnitude floating-point values by printf suffers a loss

   of precision due to the algorithm used.

2) uClibc's printf is much stricter than glibcs, especially regarding positional

   args.  The entire format string is parsed first and an error is returned if

   a problem is detected.  In locales other than C, the format string is checked

   to be a valid multibyte sequence as well.  Also, currently at most 10 positional

   args are allowed (although this is configurable).

3) BUFSIZ is configurable, but no attempt is made at automatic tuning of internal

   buffer sizes for stdio streams.  In fact, the stdio code in general sacrifices

   sophistication/performace for minimal size.

4) uClibc allows glibc-like custom printf functions.  However, while not

   currently checked, the specifier must be <= 0x7f.

5) uClibc allows glibc-like custom streams.  However, no in-buffer seeking is

   done.

6) The functions fcloseall() and __fpending() can behave differently than their

   glibc counterparts.

7) uClibc's setvbuf is more restrictive about when it can be called than glibc's

   is.  The standards specify that setvbuf must occur before any other operations

   take place on the stream.

8) Right now, %m is not handled properly by printf when the format uses positional

   args.

9) The FILEs created by glibc's fmemopen(), open_memstream(), and fopencookie()

   are not capable of wide orientation.  The corresponding uClibc routines do

   not have this limitation.

10) For scanf, the C99 standard states "The fscanf function returns the value of

    the macro EOF if an input failure occurs before any conversion."  But glibc's

    scanf does not respect conversions for which assignment was surpressed, even

    though the standard states that the value is converted but not stored.

glibc bugs that Ulrich Drepper has refused to acknowledge or comment on

  ( http://sources.redhat.com/ml/libc-alpha/2003-09/ )

-----------------------------------------------------------------------

1) The C99 standard says that for printf, a %s conversion makes no special

   provisions for multibyte characters.  SUSv3 is even more clear, stating

   that bytes are written and a specified precision is in bytes.  Yet glibc

   treats the arg as a multibyte string when a precision is specified and

   not otherwise.

2) Both C99 and C89 state that the %c conversion for scanf reads the exact

   number of bytes specified by the optional field width (or 1 if not specified).

   uClibc complies with the standard.  There is an argument that perhaps the

   specified width should be treated as an upper bound, based on some historical

   use.  However, such behavior should be mentioned in the Conformance document.

3) glibc's scanf is broken regarding some numeric patterns.  Some invalid

   strings are accepted as valid ("0x.p", "1e", digit grouped strings).

   In spite of my posting examples clearly illustrating the bugs, they remain

   unacknowledged by the glibc developers.

4) glibc's scanf seems to require a 'p' exponent for hexadecimal float strings.

   According to the standard, this is optional.

5) C99 requires that once an EOF is encountered, the stream should be treated

   as if at end-of-file even if more data becomes available.  Further reading

   can be attempted by clearing the EOF flag though, via clearerr() or a file

   positioning function.  For details concerning the original change, see

   Defect Report #141.  glibc is currently non-compliant, and the developers

   did not comment when I asked for their official position on this issue.

6) glibc's collation routines and/or localedef are broken regarding implicit

   and explicit UNDEFINED rules.

More to follow as I think of it...